The invention relates to a shielded cable and more particularly, to a shielded drop cable for the transmission of RF signals.
In the transmission of RF signals such as cable television signals, cellular telephone signals, and data, a drop cable is generally used as the final link in bringing the signals from a trunk and distribution cable directly into a subscriber""s home. Conventional drop cables include an insulated center conductor that carries the signal and a conductive shield surrounding the center conductor to prevent signal leakage and interference from outside signals. In addition, the drop cable generally includes a protective outer jacket to prevent moisture from entering the cable. One common construction for drop cable includes an insulated center conductor, a laminated tape formed of metal and polymer layers surrounding the center conductor, a layer of braided metallic wires, and an outer protective jacket.
It has been found during the manufacture of conventional drop cables, that the relatively small diameter round wires forming a typical braided covering will easily break unless the braiding is done at a relatively slow speed. For example, the braiding operation may typically be performed at a rate of only about 10 to 11 linear feet per minute. In contrast, the final step of applying the protective plastic jacket can be performed at speeds as high as 450 linear feet per minute. Moreover, proper extrusion of the plastic jacket requires a higher linear speed than 10 to 11 feet per minute. Thus, two discrete process steps are required to form the braid and then apply the outer protective plastic jacket in a conventional drop cable manufacturing process.
In addition to process concerns, the cost of the raw material for making a coaxial drop cable is often an important factor in the cable design. For a cable television company having thousands of miles of drop cable, the cost savings of a minor reduction in the amount of material in the drop cable becomes significant. Unfortunately, it is not possible to reduce the amount of metal in the round reinforcing wire covering of the prior art drop cable without compromising the strength of the cable or without further reducing the speed of the braiding step.
The shielding of the center conductor is another important aspect of the cable design. It is generally desirable to increase the percentage of coverage that the reinforcing layer provides to the electrically conductive foil shield to thereby reduce leakage of the high frequency of signals from the cable. In a conventional round wire reinforcing covering, an increase in the desired coverage would require a greater quantity of metal and, therefore, add to the overall expense of the cable.
One approach to reducing the cost of a drop cable while providing the desired flexibility and shielding is described in U.S. Pat. No. 5,254,188 to Blew. Blew uses a coaxial cable wherein the outer conductor includes a plurality of flat reinforcing wires wrapped around a foil shield to form an electrically conductive reinforcing covering. Although Blew""s approach provides a coaxial drop cable with certain advantages, there is a desire in the art to further increase the flexibility and cost in the production of coaxial cables while maintaining the desired amount of shielding coverage.
The present invention provides a low cost, shielded coaxial drop cable having excellent flexibility and shielding coverage. The shielded coaxial cable of the invention comprises an elongate center conductor, a dielectric layer surrounding the center conductor, an electrically conductive shield surrounding the dielectric layer, a first plurality of elongate wires surrounding the electrically conductive shield, and a protective jacket surrounding the plurality of elongate wires. In accordance with the invention, the elongate wires have an elliptical cross section with a major axis and a minor axis wherein the major axis to minor axis ratio is from greater than 1:1 to less than 5:1.
The coaxial cables of the invention produce excellent shielding but use less material than conventional cables that use elongate wires having a circular cross section. Thus, the present cables are less expensive to produce than conventional cables. The elongate strands used in the invention also have good tensile strength and are not subject to breakage even at high production speeds (e.g. 200 ft/min or more). Furthermore, the elongate wires because of their elliptical cross section are freely displaceable axially and capable of slipping over or under one another. As a result, the cables of the invention have excellent flexibility. In addition, the wires can be easily processed using conventional machinery. Moreover, the elongate wires of the invention can be readily formed into braids. The cables of the invention can also be easily connectorized using standard connectors.
In a preferred embodiment of the invention, the elongate wires have a major axis to minor axis ratio of from 1.5:1 to 3:1, more preferably of about 2:1. The first plurality of elongate wires is preferably arranged such that the surfaces corresponding to the major axes of the elongate wires contact the underlying metallic shield. In addition, the elongate wires are preferably helically arranged around the underlying electrically conductive shield. The coaxial cable can also include a second plurality of elongate wires helically arranged about the first plurality of elongate wires and having a helical orientation opposite the orientation of the first plurality of elongate wires. The first plurality of elongate wires can also be interlaced with a second plurality of elongate wires to form a braid around the first electrically conductive shield. In either case, the second plurality of elongate wires preferably has an elliptical cross section with a major axis to minor axis ratio of from greater than 1:1 to less than 5:1. The first plurality and second plurality of elongate wires are preferably formed from aluminum or an aluminum alloy, or copper or a copper alloy.
Furthermore, in the preferred embodiment of the invention, the electrically conductive shield extends longitudinally along the cable and more preferably has overlapping longitudinal edges. Preferably, the electrically conductive shield comprises a bonded metal-polymer-metal laminate tape. In addition, the electrically conductive shield is preferably adhesively bonded to the dielectric layer and the dielectric layer is adhesively bonded to the center conductor.
The invention further includes a method of making the shielded cables of the invention. The method includes advancing a center conductor along a predetermined path of travel, applying a dielectric layer around the center conductor, applying a electrically conductive shield around the dielectric layer, arranging a plurality of elongate wires having an elliptical cross section with a major axis to minor axis ratio of from greater than 1:1 to less than 5:1 around the electrically conductive shield, and applying a cable jacket around the plurality of elongate wires. The elongate wires preferably have a major axis to minor axis ratio of from 1.5:1 to 3:1, more preferably of about 2:1. The elongate wires are preferably helically arranged around the underlying electrically conductive shield. A second plurality of elongate wires can also be helically arranged around the first plurality of elongate wires using a helical orientation opposite the orientation of the first plurality of metal wires, or braided with the first plurality of elongate wires around the electrically conductive shield. The second plurality of elongate wires preferably has an elliptical cross section with a major axis to minor axis ratio of from greater than 1:1 to less than 5:1. The electrically conductive shield is preferably longitudinally arranged around the dielectric layer, more preferably by overlapping the longitudinal edges of the electrically conductive shield.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawings, which describe both the preferred and alternative embodiments of the present invention.